For many years, telecommunications networks have used fixed hierarchical routing (FHR), which is still the most common form of call routing used by North American telephone companies. In 1981, however, Northern Telecom introduced Dynamically Controlled Routing (DCR) which makes use of the capabilities of modern stored program control switches with high-availability, real-time data processing abilities and the "intelligence" of a central computer, typically called the network processor (NP). Periodically, say every ten seconds, the switches report to the network processor the status of their trunk groups, using a data communications system comprising X-25 links. The network processor uses trunk group idleness information from the switches to update tables of alternate routing information kept by the switches. When a switch is unable to route a call because the selected direct link to the destination switch is blocked or unavailable, the switch attempts an alternative route obtained from its table of alternative routes.
For more information about dynamic routing, as compared with FHR, the reader is directed to articles entitled "Dynamically Controlled Routing" by Hugh Cameron and Serge Hurtubise, in Telesis, Vol 1, 1986, published by Bell-Northern Research and "State-Dependent Dynamic Traffic Management for Telephone Networks", by Jean Regnier and W. Hugh Cameron, IEEE Communications Magazine, October 1990, pp. 42-53. An early version of dynamically controlled routing is disclosed in U.S. Pat. No. 4,284,852 issued August 1981 and refinements are described in U.S. Pat. No. 5,526,414 issued December 1994. All four of these documents are incorporated herein by reference.
DCR has been followed by other networks which employ dynamic routing, including Dynamic Alternate Routing (DAR) by British Telecom and Real-Time Network Routing (RTNR) by AT&T Bell Laboratories, while it is reported that France Telecom have conducted trials of a STAR (System for Testing Adaptive Routing) and Bell Communications Research has conducted trials of DR5 (Dynamic Routing with 5 minute updates).
Dynamic routing yields significant advantages in network control. It ensures the efficiency of switching equipment by preventing excessive traffic from reaching the machine; inhibits the spread of switching congestion; maintains circuit group efficiency by reducing the number of links per call when facilities are congested; and makes full use of available idle facilities. Despite these advantages, dynamic routing systems are not widely used. One reason is that presently-available dynamic routing systems are not compatible with those of other manufacturers. Since most telephone companies use switching equipment from different manufacturers, dynamic routing cannot be deployed in their networks.
In recent years, so-called "intelligent networks"(IN) have been developed and are beginning to be implemented. Intelligent networks are intended to facilitate the development of a wide range of network-wide services. In an intelligent network, each switching element may interrupt call processing and exchange messages with a central computer to obtain instructions for completing the call. The messages are exchanged by way of a data communications system and must comply with existing and evolving standards and protocols, regardless of who made the switching equipment deployed in the network.
"Intelligent Networks"(IN) employ out-of-band signalling systems such as Signalling System No. 7 (SS7), also known as Common Channel Signalling No. 7 (CCS7). Such signalling systems exchange Transaction Capability Application Part (TCAP) messages or queries between network elements and the central computer to deploy selected services. In addition, the SS7 system carries ISDN-User Part (ISUP) messages between network node switching units to set up and route calls. Hence, the TCAP and ISUP messages are handled by a data communications system separate from the trunks which carry the calls themselves.
Similarities between Intelligent Networks and centralized Dynamic Routing networks, such as the use of a remote computer database and associated communication facilities, have led to a proposal to implement dynamic routing in such intelligent networks. Such proposal, entitled "A Dynamic Routing System Based on the Intelligent Network (IN) Architecture" by G. Fischer, J. Rammer and K. Hofmann was presented at ISS '95. Limitations of this proposal by Fischer et al, however, appear to be that it requires the switching units to be modified and cannot readily be implemented in networks which use switching elements from different manufacturers. Moreover, the proposal does not satisfactorily address limitations of DR systems resulting from anti-looping measures to prevent routing of a call back to a node that it had visited previously.
In order to ensure that looping does not occur, Nortel's DCR, and other dynamic routing schemes, limit the alternate route to a maximum of two links per call overflowing the direct link to the destination. At the tandem node, the dynamic routing scheme does not allow the call to obtain a second alternate route. If the second link of the alternate route is blocked or unavailable, the tandem node can only block the call or use exception routing to route it out of the DCR network. Such a limitation is undesirable since, with updates every ten seconds, there is a distinct possibility that the second link will be blocked. Reducing the update interval is not a satisfactory solution.